fs: Fix return type of do_mount() from long to int

fs: Fix return type of do_mount() from long to int

Fix the return type of do_mount() function from long to int to match its ac
tual behavior. The function only returns int values, and all callers, inclu
ding those in fs/namespace.c and arch/alpha/kernel/osf_sys.c, already treat
 the return value as int. This change improves type consistency across the
filesystem code and aligns the function signature with its existing impleme
ntation and usage.

Signed-off-by: Sentaro Onizuka <sentaro@amazon.com>
Link: https://lore.kernel.org/r/20250113151400.55512-1-sentaro@amazon.com
Signed-off-by: Christian Brauner <brauner@kernel.org>