Document the new mnt_ns_fd parameter to struct mnt_id_req.

req.mnt_ns_fd can be used to query for a mount in a foreign mount
namespace.

The mnt_ns_fd parameter description is based on this commit message [1].

Link [1]:
<https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=9020d0d844ad58a051f90b1e5b82ba34123925b9>

Signed-off-by: Bhavik Sachdev <redacted>
---
 man/man2/statmount.2 | 21 ++++++++++++++++++++-
 1 file changed, 20 insertions(+), 1 deletion(-)

diff --git a/man/man2/statmount.2 b/man/man2/statmount.2
index 0b906a334..e835eb0f6 100644
--- a/man/man2/statmount.2
+++ b/man/man2/statmount.2

@@ -24,6 +24,7 @@ .SH SYNOPSIS
 .EX
 .B struct mnt_id_req {
 .BR "    __u32  size;" "        /* sizeof(struct mnt_id_req) */"
+.BR "    __u32  mnt_ns_fd;" "   /* fd of mnt_ns to query the mnt_id in */"
 .BR "    __u64  mnt_id;" "      /* The mnt_id being queried */"
 .BR "    __u64  param;" "       /* An ORed combination of the STATMOUNT_ constants */"
 .BR "    __u32  mnt_ns_id;" "   /* The id of mnt_ns to query the mnt_id in */"

@@ -74,7 +75,9 @@ .SH DESCRIPTION
 To access a mount's status,
 the caller must have CAP_SYS_ADMIN in the user namespace.
 In case of accessing a mount in a foreign mount namespace (specified via
-.IR req.mnt_ns_id ),
+.I req.mnt_ns_id
+or
+.IR req.mnt_ns_fd ),
 the foreign mount namespace should be child of the current namespace.
 .P
 This function returns information about a mount,

@@ -97,6 +100,15 @@ .SS The mnt_id_req structure
 it should always be set to
 .IR sizeof(struct\~mnt_id_req) .
 .P
+.I req.mnt_ns_fd
+can be obtained from
+.B PIDFD_GET_MNT_NAMESPACE
+.BR ioctl (2)
+operation or by opening a file descriptor to
+.IR /proc/[pid]/ns/mnt

+and is used to specify a foreign mount namespace in which to query
+.IR req.mnt_id .
+.P
 .I req.mnt_id
 can be obtained from either
 .BR statx (2)

@@ -365,6 +377,13 @@ .SH ERRORS
 .I req
 is of insufficient size to be utilized.
 .TP
+.B EINVAL
+Both
+.I req.mnt_ns_id
+and
+.I req.mnt_ns_fd
+were set.
+.TP
 .B E2BIG
 .I req
 is too large.

-- 
2.53.0