fs/statmount: fix slab out-of-bounds write in statmount_mnt_idmap

fs/statmount: fix slab out-of-bounds write in statmount_mnt_idmap

statmount_mnt_idmap() writes one mapping with seq_printf() and then
manually advances seq->count to include the NUL separator.

If seq_printf() overflows, seq_set_overflow() sets seq->count to
seq->size. The manual seq->count++ changes this to seq->size + 1.
seq_has_overflowed() then no longer detects the overflow. The corrupted
count returns to statmount_string(), which later executes:

    seq->buf[seq->count++] = '\0';

This causes a 1-byte NULL out-of-bounds write on the dynamically
allocated seq buffer.

Fix this by checking for overflow immediately after seq_printf().

Fixes: 37c4a9590e1e ("statmount: allow to retrieve idmappings")
Signed-off-by: Junyoung Jang <graypanda.inzag@gmail.com>
Link: https://patch.msgid.link/20260504112649.1862936-1-graypanda.inzag@gmail.com
Signed-off-by: Christian Brauner <brauner@kernel.org>