fscrypt: optimize fscrypt_initialize()

fscrypt: optimize fscrypt_initialize()

fscrypt_initialize() is a "one-time init" function that is called
whenever the key is set up for any inode on any filesystem.  Make it
implement "one-time init" more efficiently by not taking a global mutex
in the "already initialized case" and doing fewer pointer dereferences.

Link: https://lore.kernel.org/r/20230406181245.36091-1-ebiggers@kernel.org
Signed-off-by: Eric Biggers <ebiggers@google.com>