fs/fs_context: Mark an unlikely if condition with unlikely() in vfs_parse_monolithic_...

fs/fs_context: Mark an unlikely if condition with unlikely() in vfs_parse_monolithic_sep()

There is no mount option with pattern "...,=key_or_value,...", so the if
condition '(value == key)' in while loop of vfs_parse_monolithic_sep() is
is unlikely true.

Mark the condition with unlikely() to improve both performance and
readability.

Signed-off-by: Zijun Hu <quic_zijuhu@quicinc.com>
Link: https://lore.kernel.org/20250410-fix_fs-v1-5-7c14ccc8ebaa@quicinc.com
Signed-off-by: Christian Brauner <brauner@kernel.org>